The History of Encrypted Server Name Indication (SNI) to ESNI to ECH
Server Name Indication (SNI) is a feature in the Transport Layer Security (TLS) protocol that enables a client to send the hostname of the website it wants to connect to before starting the SSL/TLS negotiation. This information is used by the server to determine which certificate to present to the client, allowing multiple websites to share the same IP address and still maintain secure, encrypted connections.
However, before the advent of encrypted SNI, this information was sent in plain text, making it vulnerable to snooping and tampering.